This invention relates to an inverter system which is suitable as a power supply system for electronic data processing systems and for electronic systems in general, and more particularly, to an improvement for a control device which limits overcurrent in power semiconductor switches.
FIG. 1 shows the power circuit in a prior art inverter. In the Figure, the letter E denotes a DC power source, and X, Y are DC source terminals. S.sub.1 to S.sub.4 denote power semiconductor switches which may be, for instance, transistors, or GTO thyristors. The portion I encircled by a broken line is a DC to AC converter. The letter F denotes a filter which extracts a sine wave which is the fundamental frequency component of the distorted AC voltage in the output of the DC to AC converter I. The letter L denotes a load of the inverter formed by the DC to AC converter and the filter F. T is a transformer and CT is a current transformer for current sensing.
A current suppressing control for limiting overcurrent in such a device is constructed to control current at a constant value by interrupting a feedback voltage control system in such a manner that the output current sensed by the current transformer CT will not exceed a preset value.
FIG. 2 is a block diagram of a prior art inverter feedback control system wherein the reference numeral 1 denotes a reference DC voltage generator which generates a DC voltage as a reference against which the output voltage is compared. A rectifier and smoothing filter 2 converts the output voltage v of the inverter system into a DC voltage. An error amplifier 3 amplifies the difference between the output of the reference DC voltage circuit 1 and the output of the rectifier and smoothing filter 2. A reference DC voltage generator 1' generates a DC voltage which is the reference for the output current limiting system. A peak voltage holding circuit 2' is internally provided with a time constant circuit which converts the output current i to direct current, as well as, make the output voltage rise slowly after the over-current has disappeared. An error amplifier 3' amplifies the difference between the output of the reference voltage generator 1' and the output of the peak voltage holding circuit 2'. The reference numeral 4 denotes a sine wave generator. An amplitude modulator 5 modulates the output amplitude of the generator 4 by using either the level of the output from the error amplifier 3 or that of the output from the error amplifier 3', whichever is lower. A triangle voltage generator 6 and a PWM (pulse width modulation) circuit 7 generates a pulse width modulation signal by using the output from the triangle voltage generator 6 as a carrier and by receiving the output from the modulator 5 as a modulation input. The reference numeral 8 denotes a logic circuit which converts the output from the PWM circuit 7 into an on-off driving signal for switches which turn on/off the output of the PWM circuit 7.
In the prior art system mentioned above, when an overcurrent occurs temporarily in the output current and after the overcurrent is disappeared, the output voltage must be increased at a sufficiently slow rate so as not to saturate the transformer T. More particularly, even if the overcurrent flows only for a short time (e.g. a half cycle) and the load is restored immediately to a normal condition, the resulting abnormally low voltage will last over several cycles and the output voltage from the inverter will not immediately return to the normal voltage level. Meanwhile, power is not fed to the load causing disruption to the load especially when a data processing system is connected as a load.
Accordingly, this invention obviates the aforementioned prior art defects and provides an inverter device which is capable of restoring the output voltage to a normal value immediately after the overcurrent disappears even if such an overcurrent occurs temporarily and does not influence other loads even in the case where an overcurrent of short duration, such as a rush current for a transformer or filter capacitors, flow in the load.